2.5. Use of micro-algae in aquaculture

2.5.1. Bivalve molluscs
2.5.2. Penaeid shrimp
2.5.3. Marine fish

Micro-algae are an essential food source in the rearing of all stages of marine bivalve molluscs (clams, oysters, scallops), the larval stages of some marine gastropods (abalone, conch), larvae of several marine fish species and penaeid shrimp, and zooplankton.

2.5.1. Bivalve molluscs

Intensive rearing of bivalves has so far relied on the production of live algae, which comprises on average 30% of the operating costs in a bivalve hatchery. The relative algal requirements of the various stages of the bivalve culture process depend on whether the operation aims at the mass-production of larvae for remote setting or growing millions of seed till planting size. In either case, the juveniles, representing the largest biomass in the hatchery and demanding the highest weight-specific rations, consume the largest volumes of algal culture (Fig. 2.16.). The algal species that were reported in an international survey among hatchery operators in 1991 are listed in Table 2.14. Eight algal species (Isochrysis sp., clone T-Iso; C. gracilis; C. calcitrans; T. suecica; T. pseudonana, clone 3H; P. lutheri; I. galbana; S. costatum) were widely used and represented over 90% of the volume of algal culture produced in 23 facilities.

Figure 2.16. Requirements for cultured algae in hatchery and nursery culture of bivalve molluscs (Utting and Spencer, 1991).

Table 2.14. Algal species used in hatchery and nursery rearing of bivalve molluscs as reported in an international questionnaire. Species are ranked according to decreasing frequency of use (Coutteau and Sorgeloos, 1992).

Algal species	frequency of use†	total daily production n‡ volume (m3)
Isochrysis sp., clone T-Iso	31	18	23.8
Chaetoceros gracilis	23	11	14.1
Chaetoceros calcitrans	16	10	6.0
Tetraselmis suecica	15	10	39.1
Thalassiosira pseudonana, clone 3H	14	9	112.0
Pavlova lutheri	11	7	11.7
Isochrysis galbana	8	5	9.1
Skeletonema costatum	6	3	58.8
Chroomonas salina	5	3	0.76
Dunaliella tertiolecta	4	2	2.2
Chaetoceros simplex	3	3	1.76
Chaetoceros muelleri	3	2	5.0
Nannochloropsis sp.	3	2	0.20
Cyclotella sp.	2	1	0.36
Phaeodactylum tricornutum	2	1	2.0
Tetraselmis chui	2	0	-
Pavlova salina	1	1	3.18
Dicruteria sp.	1	1	4.07
Tetraselmis levis	1	0	-
Dunaliella perva	1	1	0.012
Thalassiosira weissfloggii	1	1	0.12
Chlamydomonas sp.	1	1	0.52
Chlorella sp.	1	1	0.36
TOTAL	43	23	295

†: number of hatcheries growing each algal species (from 43 completed forms)

‡: number of hatcheries providing data which allowed to calculate daily production per algal species (from 23 completed foms)

The larvae of most bivalve species have similar food preferences; suitable algal species including C. calcitrans, T. pseudonana (3H), I. galbana, and T. suecica (for larvae > 120 µm in length). Combinations of flagellates and diatoms provide a well balanced diet which will generally accelerate the rate of larval development to metamorphosis in comparison with unialgal diets. The quantity fed depends upon the larval density, but suitable cell concentrations (expressed as cells.µl^-1) are given by each of the following combinations:

· I. galbana; 50
· C. calcitrans; 250
· I. galbana/C. calcitrans; 25/125
· I. galbana/C. calcitrans/T. suecica; 33/83/3.3 (larvae > 120µm)

Because of the high cost of cultured algae, bivalve hatcheries prefer to move juveniles to outdoor nursery systems at a maximum size of 1-2 mm length. In this way, the duration of the juvenile phase in closely controlled hatchery conditions is relatively short for oysters at about 20 days but much longer for the slower growing clams at up to 60 days. Bivalve food rations are preferentially expressed as daily weight-specific rations, such as number of cells or percent dry weight of algae per live weight of bivalves. Seed growth is largely influenced by food ration and the optimal ration for maximum growth depends upon the species and culture conditions of the algae making up the diet, and the bivalve culture conditions. Under practical hatchery conditions, high food rations are often fed, which may be as high as 5-6% dry weight of algae per live weight of spat per day.

2.5.2. Penaeid shrimp

A typical algal feeding regime for penaeid larvae is given in Table 2.15. Algae are added during the non-feeding nauplius stage so that algae are available immediately upon molting into the protozoea stage. Algal species most often used are Tetraselmis chui, Chaetoceros gracilis, and Skeletonema costatum. As feeding preference changes from primarily herbivorous to carnivorous during the mysis stages, the quantity of algae is reduced. Nevertheless, a background level of algae is maintained as this may stabilize water quality. The “same-tank method”, in which the algae are cultured in the same water as that of the larvae using sunlight and fertilizers, was originally developed in Japan for culturing larval Penaeus japonicus and is extensively described by Liao et al. (1993).

Table 2.15: Typical algal feeding regimes (cells.ml^-1) for penaeid larvae (N: nauplius, P: protozoea, M: mysis, PL: postlarva stage) (modified from Smith et al., 1993b).

Substage	Chaetoceros neogracile (C. gracilis)	Tetraselmis chuii
N5 or N6	60,000	0-15,000
P1	100,000-120,000	30,000
P2	120,000	35,000
P3	120,000	35,000
M1	100,000	30,000
M2	75,000	20,000
M3	50,000-75,000	20,000
PL1 to PL5	20,000-75,000	5,000-20,000

2.5.3. Marine fish

Apart from the requirement for micro-algae for culturing and/or enriching live prey organisms such as Artemia and rotifers (see Chapters 3. and 4.3.), algae are often used directly in the tanks for rearing marine fish larvae. This “green water technique” is part of the commonly applied techniques for rearing larvae of gilthead seabream Sparus aurata (50,000 cells ml^-1 of Isochrysis sp. + 400,000 cells.ml^-1 of Chlorella sp. per day), milkfish Chanos chanos (between 500 and 3,500 Chlorella cells.ml^-1 are added from hatching till day 21), Mahimahi Coryphaena hippurus (200,000 cells.ml^-1 of either Chaetoceros gracilis, Tetraselmis chui, or Chlorella sp.), halibut Hippoglossus hippoglossus (Tetraselmis sp.), and turbot Scophthalmus maximus (60,000 cells.ml^-1 of Tetraselmis sp. or 130,000 cells.ml^-1 of I. galbana).

The effects of the presence of micro-algae in the larval rearing tank are still not fully understood and include:

· stabilizing the water quality in static rearing systems (remove metabolic by-products, produce oxygen);

· a direct food source through active uptake by the larvae with the polysaccharides present in the algal cell walls possibly stimulating the non-specific immune system in the larvae;

· an indirect source of nutrients for fish larvae through the live feed (i.e. by maintaining the nutritional value of the live prey organisms in the tank);

· increasing feeding incidence by enhancing visual contrast and light dispersion, and

· microbial control by algal exudates in tank water and/or larval gut.